Adsorption

Adsorption

The phenomenon of attracting and retaining the molecules of the substance on the surface of a liquid or a solid resulting into a higher concentration of molecules on the surface is called adsorption. The substance thus adsorbed on the surface is called the adsorbate and the substance on which it is adsorbed is called adsorbent. The reverse process, i.e. removal of the adsorbed substance from the surface is called desorption (which can be brought about by heating or reducing the pressure). The adsorption of gases on the surface of metals is called occlusion.

Difference between adsorption and absorption:

Positive and negative adsorption

When the concentration of the adsorbate is more on the surface of the adsorbent than in the bulk, it is called positive adsorption. If the concentration of the adsorbate is less, relative to its concentration in the bulk, it is called negative adsorption.

Types of adsorption

An experimental study of the adsorption of various types on solids shows that there are two main types of adsorption. Physical adsorption & chemical adsorption.

Physical adsorption & chemical adsorption

Difference between physical adsorption and chemisorption

Factors Affecting the Adsorption of Gases

Almost all solids adsorb gases to some extent, however, the exact amount of a gas adsorbed depends upon the following factors.

(i) Nature and surface area of the adsorbent: The greater the surface area of the adsorbent, greater is the volume of the gas adsorbed. Therefore, charcoal and silica gel are excellent adsorbents because they have highly porous structures and hence large surface areas.

(ii) Nature of the gas: Different gases are adsorbed to different extents by the same adsorbent at the same temperature. As critical temperature increases, ease of liquefaction increases and hence adsorption increases.

(iii) Effect of temperature: Adsorption is generally temperature dependent. Mostly, adsorption is an exothermic process and therefore, adsorption decreases with increasing temperature. However, as expected for endothermic adsorption processes, adsorption increase with increase in temperature.

(iv) Pressure: At constant temperature, the adsorption of a gas increases with increase of pressure. It is observed that at low temperature, the adsorption of a gas increases very rapidly as the pressure is increased.

(v) Activation of the solid adsorbent: This is usually done by increasing the surface area of the adsorbent by any of the following ways,

(a) making the surface of the adsorbent rough.

(b) subdividing the adsorbent into smaller piece or grains.

(c) removing the gases already absorbed.

Freundlich's Adsorption Isotherm

In case of adsorption of gases on solids, the relation between x/m and the pressure p of the gas at constant temperature is given by the equation.

$\frac{x}{m}=K{p}^{1/n}\left(n>1\right)...\left(i\right)$

where K and n are the parameter of the equation depending upon the nature of the gas and the solid. According to this, x/m increases with increase of p but since n > 1, x/m does not increase as rapidly as P, as can be seen from the isotherm.

Taking logarithms on both sides of equation (i) we get,

$\log \frac{x}{m}=\log K+\frac{1}{n}\log p...(ii)$

Thus, if we plot a graph between log(x/m) and log p, a straight line is obtained. The slope of the line is equal to 1/n and the intercept on log (x/m) axis will correspond to logK. Therefore, value of K and n can be determined.

Langmuir Adsorption Isotherm

One of the drawbacks of the Freundlich adsorption isotherm is that it fails at high pressure. Langmuir's adsorption isotherm is based on kinetic theory of gases. Langmuir considered adsorption to consist of the following two opposing processes:

(i) adsorption of the gas molecules on the surface of the solid.

(ii) desorption of the adsorbed molecules from the surface of the solid.

Langmuir believed that eventually a dynamic equilibrium is established between the above two opposing processes. He also assumed that the layer of the adsorbed gas was

unimolecular. Such type of adsorption is obtained in the case of chemisorption, hence isotherm works particularly well for chemisorption.

The Langmuir adsorption isotherm is represented by the relation,

$\frac{x}{m}=\frac{ap}{1+bp}...\left(iii\right)$

Where a and b are two Langmuir parameters. At very high pressure, the above isotherm acquires the limiting from,

$\frac{x}{m}=\frac{a}{b}\left(atveryhighpressure\right)1+bp\simeq bp...\left(iva\right)$

At very low pressure, equation (iii) is reduced to x/m = ap

(at very low pressure) 1+ bp

In order to determine the parameters a and b, equation (iii) may be written in its inverse form.

$\frac{m}{x}=\frac{1+bp}{ap}=\frac{b}{a}+\frac{1}{ap}...\left(v\right)$

A plot of m/x against 1/p gives a straight line with slope and intercept equal to 1/a and b/a, respectively, thus both parameters can be determined.

Langmuir isotherm indicates that at low pressure x/m increases linearly with p. At high pressure, x/m becomes constant, i.e. the surface is fully covered and change in pressure has no effect and no further adsorption takes place, as is evident from figure.